The present invention relates generally to antenna architectures and, more particularly, to slot antenna structures utilizing a coaxial resonator for use with mobile communication units including, but not limited to, cellular radiotelephone handsets in wireless communications systems.
Wireless telecommunication systems are known to employ portable or handheld mobile communication units such as for example cellular radiotelephone handsets operatively associated with a limited number of wireless communication resources and a remote system resource controller. As the cellular handsets require smaller size and less thickness, miniaturization or "down-sizing" of the antenna module adapted for use with such units has become more critical in recent years. Until today, several approaches to the small-size antenna have been proposed and developed. One previously known approach is to use a slot antenna incorporating a coaxial resonator. An exemplary coaxial-resonator slot antenna has been disclosed in U.S. Pat. No. 5,914,693 filed Sep. 5, 1996. The slot antenna disclosed is structurally designed so that its centrally disposed elongate or "strip" conductor is kept in non-contact with a flat rectangular box-like conductive frame body of the resonator to miniaturize the antenna. This antenna also features in absence of any particular outer projections facilitating mounting of the antenna into the enclosure of a cellular radiotelephone handset.
Since the prior art small-size slot antenna has such resonator structure, the volume thereof is proportional to its impedance matching bandwidth--that is, the smaller the volume, the less the bandwidth. Accordingly, in cases where the antenna is practiced at communication units of a broad-band wireless communication system with an increased capacity by use of a plurality of different carrier frequencies allocated thereto, the impedance matching frequency band to be achieved by the antenna might be widened enlarging the antenna in size and in volume.
As readily appreciated by a person skilled in the art, those frequencies used for telephone interconnect calls between one particular base station and its associated wireless communication units, such as cellular handsets, are much less than the frequency band inherently allocated to the entire communication system. Accordingly, adaptively changing the antenna's impedance matching center frequency to a presently selected frequency for a telephone call attempt may render narrower the antenna's inherent frequency band, which in turn downsizes the antenna. One typical antenna module incorporating this approach is a tunable slot antenna as disclosed in copending U.S. patent application (Ser. No. 09/035,848, filed Mar. 6, 1998. This antenna is a coaxial resonator-used slot antenna including a variable capacitive element connected between a selected location at or near one end of a built-in strip conductor, which end is far from a connection point of the strip being supplied with high-frequency electric power, and an opposing plate of a rectangular box. The antenna may vary in impedance matching center frequency by varying the capacitance value of the variable capacitance element. An exemplary structure of the tunable slot antenna is shown in FIGS. 10a and 10b.
The tunable slot antenna shown in FIGS. 10a-10b includes a conductive flat box 1. The box 1 has therein an elongate strip-like conductor 3 that is electrically insulated from the box 1 and extends along the axis of resonance. The box 1 has a top plate surface in which a slot 2 is formed overlying and crossing the strip 3. Strip 3 has one end at which a connection point 10 is disposed and its opposite end has a small opening 11 defined for disposal of an island conductor 4. The connection point 10 is operatively coupled to a high frequency or radio frequency (RF) power supply circuit 7, which operates to supply RF power between the connection point 10 and its opposing part of the bottom plate of the box 1. RF power supply 7 is associated with a certain element for elimination of unwanted high frequency current drain, and a variable direct current (DC) power supply 9. A variable capacitive element 6 is connected between a selected location at or near the far end of strip 3 with small hole 11 and its opposing part of the top plate of the box 1. Variable capacitor 6 receives a DC voltage from DC power supply 9 via strip 3 and RF current drain eliminator 8.
In the antenna structure of FIGS. 10a-10b, the variable capacitor 6 may vary in capacitance value in response to receipt of a DC voltage applied from variable DC power supply 9 thereby causing a current flowing in strip 3 just beneath slot 2 to likewise change in phase. Such strip current phase change may in turn serve to permit strip 3 to change in length equivalently or "virtually," which length closely relates to the resonant frequency of the tunable slot antenna shown. This makes it possible for the antenna to change or modify the impedance matching center frequency, that is, resonant frequency.